1. Field of the Invention
The present invention relates to a rolling bearing and, in particular, to a rolling bearing such as a bearing for iron and steel represented by a roll neck bearing which, even under a severe using condition of a high load and a high surface pressure, does not produce an unexpected short life product but can be guaranteed to have a stable life.
2. Related art
Recently, a bearing using environment has been severer and severer and, in a bearing to be used in iron and steel facilities, a load and a surface pressure to be applied to the bearing have been higher and higher. In such severe condition, as the bearing to be used in iron and steel facilities, there is more and more strongly required a bearing which not only can provide a long life but also can stamp out a short-life bearing occurring unexpectedly. The reason for this requirement is as follows: that is, in a continuous production system in which the inspection and maintenance of a production line are carried out every preset given time, if there occurs a short-life bearing which comes short of the given operating time, then the production line is caused to stop within the operating time, which incurs great damage.
As one of factors that block the durability of the bearing, there is found a defect or damage in the material of the bearing. For steel material used to produce a bearing, recently, there has been employed a method in which, after the steel material is rolled into a steel bar, for example, in a steel making process, all steel bars are checked for internal defects thereof using ultrasonic waves or the like (see Special Steel, vol. 46, No. 6, page 31, edited by Special Steel Club Co.). The main object of this inter-process defect checking method is to detect a hole (a defect) existing in the inside of the steel material, such as a macro-streak-flaw, a blow hole, or an unpressed portion left in a rolling and forging operation, or the like; and, this method uses a low frequency in the range of 2-5 MHz for detection of the hole or defect. Thanks to this method, there has been eliminated a large-sized defect of the order of several mm. However, in the steel material that has been rolled but left as it is, the crystal grain of the inside thereof and the surface layer thereof are rough, thereby causing the noise to become large, which in turn makes it impossible to detect the defect with high accuracy.
On the other hand, it is known that a non-metallic inclusion (intervening material) existing in the material of a bearing has a great influence on the life of the bearing itself. For example, in steel making facilities, a backup roll bearing is used under a high load and in a well-lubricated condition given mainly by oil lubrication; and, when the bearing is used in such lubrication condition, it has been recently found that, if any non-metallic inclusion exists in the vicinity of the surface layer portion of the raceway surface of an inner or outer race of the bearing, then such non-metallic inclusion causes a defect such as a crack or the like in the bearing, thereby reducing the life of the bearing.
In order to avoid the above problem, recently, there have been proposed many proposals each of which specifies the number of hard inclusions (mainly, inclusions belonging to oxide-system materials consisting mainly of Al.sup.2 O.sup.3, or inclusions belonging to Ti-system materials consisting mainly of TiN) to thereby enhance the purity of the bearing greatly and thus extend the life of the bearing.
For example, according to Japanese Patent Publication No. 6-145883 of Heisei, there is disclosed a method in which highly-purified steel including within nine pieces of Al.sup.2 O.sup.3 of 10 .mu.m or more existing and within nine pieces of TiN of 5 m in an area to be checked of 320 mm.sup.2 is used to thereby be able to guarantee the long life of the bearing. As similar examples aiming at extending the life of the bearing by limiting the number of the non-metallic inclusions, there are also known Japanese Patent Publication No. 3-56640 of Heisei and Japanese Patent Publication No. 7-109541 of Heisei respectively filed by the present applicants, as well as Japanese Patent Publication No. 5-117804 of Heisei, Japanese Patent Publication No. 6-192790 of Heisei, and the like.
Every one of the technologies disclosed in the above-cited publications, when specifying the quantity of inclusions, observes a very tiny given area of, for example, 320 mm.sup.2 or 165 mm.sup.2 by a microscope or the like and specifies the purity of the steel material in accordance with the number of inclusions detected in such given area.
However, although the purity of the steel material is enhanced in the above-mentioned manner, all the products made of the steel material are not inspected and verified for the number of inclusions. In other words, everyone of the above technologies, simply by observing the very tiny given area of the bearing material representatively, evaluates the purity of the whole bearing and the bearing material.
The present inventors have taken up this problem and made every effort to solve this problem. For example, when many raceway surface layer portions of inner and outer races of bearings for backup rolls used in a rolling mill were checked by an ultrasonic detecting method, it has been found that, even in bearings the purity of which was found satisfactory in a sample evaluation through observations using a microscope, on quite rare occasions, there can be found a bearing in which an inclusion of as large as several hundreds .mu.m exists in the range from the raceway surface of the inner or outer race of the bearing up to the depth of several mm or so.
That is, even if a good result is obtained by checking the tiny range of an area to be checked in the top surface of the raceway surface of the inner or outer race of the bearing, it cannot be always guaranteed that an inclusion of a large size is absent in the bearing. Much less, in the case of a large-sized bearing such as a backup roll bearing or the like, a relatively large load is applied to the bearing because the area of the raceway surface thereof is large, and the depth of the portion thereof to which a stress is applied is thereby caused to increase. Therefore, simply by accurately inspecting only the raceway surface of the bearing as in the conventional method, it is difficult to eradicate a short-life bearing which can occur unexpectedly.